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<H3>fix nve/limit command 
</H3>
<P><B>Syntax:</B>
</P>
<PRE>fix ID group-ID nve/limit xmax 
</PRE>
<UL><LI>ID, group-ID are documented in <A HREF = "fix.html">fix</A> command
<LI>nve = style name of this fix command
<LI>xmax = maximum distance an atom can move in one timestep (distance units) 
</UL>
<P><B>Examples:</B>
</P>
<PRE>fix 1 all nve/limit 0.1 
</PRE>
<P><B>Description:</B>
</P>
<P>Perform constant NVE updates of position and velocity for atoms in the
group each timestep.  A limit is imposed on the maximum distance an
atom can move in one timestep.  This is useful when starting a
simulation with a configuration containing highly overlapped atoms.
Normally this would generate huge forces which would blow atoms out of
the simulation box, causing LAMMPS to stop with an error.
</P>
<P>Using this fix can overcome that problem.  Forces on atoms must still
be computable (which typically means 2 atoms must have a separation
distance > 0.0).  But large velocities generated by large forces are
reset to a value that corresponds to a displacement of length <I>xmax</I>
in a single timestep.  <I>Xmax</I> is specified in distance units; see the
<A HREF = "units.html">units</A> command for details.  The value of <I>xmax</I> should be
consistent with the neighbor skin distance and the frequency of
neighbor list re-building, so that pairwise interactions are not
missed on successive timesteps as atoms move.  See the
<A HREF = "neighbor.html">neighbor</A> and <A HREF = "neigh_modify.html">neigh_modify</A> commands
for details.
</P>
<P>Note that if a velocity reset occurs the integrator will not conserve
energy.  On steps where no velocity resets occur, this integrator is
exactly like the <A HREF = "fix_nve.html">fix nve</A> command.  Since forces are
unaltered, pressures computed by thermodynamic output will still be
very large for overlapped configurations.
</P>
<P><B>Restart, fix_modify, output, run start/stop, minimize info:</B>
</P>
<P>No information about this fix is written to <A HREF = "restart.html">binary restart
files</A>.  None of the <A HREF = "fix_modify.html">fix_modify</A> options
are relevant to this fix.
</P>
<P>This fix computes a global scalar which can be accessed by various
<A HREF = "Section_howto.html#howto_15">output commands</A>.  The scalar is the
count of how many updates of atom's velocity/position were limited by
the maximum distance criterion.  This should be roughly the number of
atoms so affected, except that updates occur at both the beginning and
end of a timestep in a velocity Verlet timestepping algorithm.  This
is a cumulative quantity for the current run, but is re-initialized to
zero each time a run is performed.  The scalar value calculated by
this fix is "extensive".
</P>
<P>No parameter of this fix can be used with the <I>start/stop</I> keywords of
the <A HREF = "run.html">run</A> command.  This fix is not invoked during <A HREF = "minimize.html">energy
minimization</A>.
</P>
<P><B>Restrictions:</B> none
</P>
<P><B>Related commands:</B>
</P>
<P><A HREF = "fix_nve.html">fix nve</A>, <A HREF = "fix_nve_noforce.html">fix nve/noforce</A>,
<A HREF = "pair_soft.html">pair_style soft</A>
</P>
<P><B>Default:</B> none
</P>
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